Various, pieces of information such as control data and image data are transmitted between a host system implemented by a data processing unit and an input/output unit implemented by, for example, a printer apparatus in an information processing equipment such as a computer. The transmission of data between such a host system and an input/output unit is dictated by a communications protocol which consists of rules and formats governing the communications therebetween. In order that the host system and input/output unit operate on such a communications protocol, the protocol must be decipherable by both the host system and the input/output unit. In this instance, it is an ordinary practice that the protocol for the input/output unit be determined or selected to conform to that adopted in the host system and, for this reason, a printer apparatus which is typical of the input/output unit is developed for exclusive use with the host system. If a protocol different from that used in the host system is to be adopted for an input/output unit, the input/output unit is required to be equipped with any protocol converter convert the protocol of the host system into the protocol used in the input/output unit.
A representative example of such a protocol converter has incorporated therein a program by means of which the control data output from the host system is to be converted into corresponding data operable in the input/output unit. The conversion program is activated by a control data processor provided in the input/output unit. In an input/output unit using such a control data processor, it is further required that the control data once converted from the protocol of the host system be further converted into the form of internal parameters effective to control the various modes and phases of operation of the printer apparatus.
A conventional protocol converter which has thus far been in use is however such that an input/output unit, to be compatible with two or more host systems, is required to incorporate protocol conversion programs corresponding in number to the host systems to which the input/output unit is to be coupled. Where it is desired that an input/output unit be connected to a larger number of host systems, the input/output unit is thus required to incorporate as many protocol conversion programs each including the larger number of steps. It is further pointed out that, when the input/output unit having such a large number of protocol conversion programs is operating with a single host system, only one of the programs is activated with the remaining programs maintained out of use. This results in a low performance efficiency in the development of the programs and in implementing the programs into the input/output unit.
A protocol conversion program tailored to the protocol used in an input/output unit may be stored in a memory device of the read only type which is detachably incorporated into the input/output unit. Such a protocol conversion program is exchangeable with another one when the input/output unit is to be used in combination with another host system. Efforts required to develop such a protocol conversion program are however not less serious than those required for the development of the conversion programs for use in an input/output unit of the type to incorporate a number of programs. Added amounts of cost and labor are further required for the fabrication of the read-only memory device. Thus, using a single input/output unit such as a printer apparatus connectable to a number of host systems has been objectionable from the viewpoint of performance efficiency.
With a view to providing a solution to such problems, a printer control system has been proposed in copending U.S. patent application Ser. No. 07/085,409 filed Aug. 14, 1987 (which corresponds to Japanese Patent Application No. 61-191683). The proposed printer control system comprises conversion means by which control data output from a data processing unit using a protocol different from the printer apparatus is to be converted into a protocol workable in the printer apparatus and control means responsive to the control data with the converted protocol for controlling the various functions of the printer apparatus. The printer control system is characterized in that the control means is provided in the form of a conversion table. Various user pps are now used in commercially available printers and include those which could not be coped with by means of such a conversion table, a typical example of such a protocol being of the type which controls a font memory.